(a) Field of the Invention
This invention relates to a composition for providing a repulpable chlorine free moisture vapor barrier coating for packaging material, especially flexible packaging material. This invention also relates to a method of coating a flexible packaging material with such composition and to repulpable packaging materials provided with such coating.
(b) Description of Prior Art
Polyethylene films and wax coatings, which are generally laminated or coated on paper and paperboard are widely used in packaging applications to protect products from moisture. However, it is well known that packaging containing polyethylene films and wax coatings are considered to be non repulpable mainly because they introduce quality problems in the fiber recovery process.
Polyethylene films and wax coatings are considered by waste dealers to be contaminant and if inadvertently used by a recovery mill, they will introduce quality problems into the fiber recovery process either by upsetting the process (plugging the screens) or contaminating the finished product.
Polyvinylidene chloride copolymer (PVdC) is another type of moisture barrier coating which provides outstanding moisture barrier and which is widely used in packaging applications. However, because of its Theological properties, PVdC does not easily form a continuous film onto the cellulose-based substrates used for packaging. Thus, to achieve the required water vapor barrier performances, the application of a heavy weight coat is necessary. However, heavy weight coat PVdC that contains chlorinated hydrocarbons has a negative impact on the environment.
There has been considerable concern about the environmental impact of chlorinated hydrocarbons. Market forces have been pressing for decreased use of chlorine and chlorine containing compound packaging. This has required a search for, alternate water vapor barrier coatings, or to decrease the amount of PVdC used in packaging.
U.S. Pat. No. 5,929,155 to Bxc3xa9rubxc3xa9 disclosed a modified PVdC composition, which permits a lighter coat weight than a regular PVdC with excellent water vapor barrier properties. In addition, packaging materials provided with such coatings are repulpable.
U.S. Pat. No. 5,562,980 to Koutitonsky discloses a multi-layer paper composition which is easily repulpable and which contains a layer of PVdC.
EP 0 718 437 A1 to Wittosh et al. discloses a repulpable paper stock provided with a base coat which contains PVdC as the main water vapor barrier ingredient.
From the prior art mentioned above, it seems that these coatings, even though they require lesser amounts of PVdC, still needs PVdC to give the necessary water vapor barrier requirement for packaging applications.
Some acrylic copolymers are also suggested in the prior art, however they have limited water vapor barrier and do not meet the necessary requirement for packaging applications. There has been a call for chlorine free paper products by environmental groups, by consumer groups, producers, retailers and government agencies. Thus, it would be desirable to have an alternative for conventional polyethylene films, wax coatings, PVdC and coatings containing PVdC that has comparable water vapor barrier properties but also has the added benefit or repulpability,
A chlorine free, wax free repulpable water vapor barrier coating should have the following essential properties:
excellent water vapor barrier property at 100% RH and 37.8xc2x0 C. (tropical conditions);
good rheological properties to achieve a continuous film on paper and paperboard to provide the necessary water vapor barrier when applied at low weight;
good water resistance;
good flexibility at room temperature;
no blocking;
be repulpable as defined in U.S. Pat. No. 5,929,155;
be based on non polluting technology which is essentially a chlorine free, wax free, formaldehyde free water based dispersion mixture
On the other hand, it is known that water vapor transmission rate takes place through a mechanism that is different than for liquid water, a water vapor barrier coating depends on a continuous film and its degree of crystallization that restricts the passage of water vapor.
The use of free chlorine compositions in providing a repulpable moisture vapor barrier is described in U.S. Pat. No. 5,897,411 which discloses a resin latex which may be carboxylated styrene butadiene and a hydrophobic component which may be mica, talc, silica, clay or kaolin. This composition does not provide a satisfactory coating on packaging material.
On the other hand, U.S. Pat. No. 6,103,809 discloses a thermoplastic composition which may be used as repulpable/recyclable adhesives and which comprises a crystalline water sensitive thermoplastic polymer blended with an amorphous water sensitive thermoplastic polymer. Tackifyier resins such as glycerol ester of hydrogenated rosin may also be included in the composition. This composition cannot be used to provide a moisture vapor barrier coating for paperboard or the like.
U.S. Pat. No. 4,284,542 discloses a hot melt adhesive composition, which includes an alkali metal ionomer of a random copolymer of ethylene methyl acrylate, a carboxylated termonomer, as well as a tackifier and a plasticizer.
U.S. Pat. No. 4,650,822 discloses an adhesive composition, which includes an elastomeric, polymeric resin, a tackifier and a terpene ether.
It is an object of the present invention to provide a repulpable, chlorine free, wax free, water vapor barrier coating for flexible packaging that is capable of commercial applications.
It is another object of the present invention to provide a repulpable coating for flexible packaging material, which is based on the combination of a tackifier and a polymer.
These and other objects of the invention may be achieved by providing a composition for providing a chlorine free vapor barrier coating for flexible packaging material, which comprises a tackifier resin, a hydrophobic chlorine free polymer having a Tg between about xe2x88x9240xc2x0 C. and 45xc2x0 C. and a minimum film forming temperature (MFFT) which is substantially the same or above the Tg of the polymer and which is capable of bonding crystallized tackifier resin to a substrate, and water in an amount sufficient to provide aqueous dispersions of the tackifier resin and the chlorine free polymer.
According to a preferred embodiment, the coating composition according to the invention may comprise a filler, which has a plate like structure, such as mica or talc, and enough water to disperse the filler in the aqueous phase. In this case, the hydrophobic chlorine free polymer must also be capable of bonding the filler.
The invention also relates to a method, which comprises coating the composition according to the invention on a flexible packaging material, and to a flexible packaging material as obtained by the method according to the invention.
Tackifier, which are either natural or synthetic resinous materials, are mainly used in the formulation of pressure sensitive adhesives to impart tack. In a lesser degree, they are also used in the formulation of other types of adhesives to impart tack as well.
According to the present invention, tackifiers are used for their tendency to crystallize. Tackifiers by themselves are unsuitable for use in water vapor barrier packaging coatings because the resulting film is either hard or brittle or soft and tacky and does not provide the necessary water vapor barrier requirement for packaging applications.
One important polymer property, which has a significant impact on the barrier performance of the coating, is the glass transition temperature (Tg) or the softness of the polymer, The Tg of the polymer defies the extent of compaction of the coating during drying. The lower the Tg of the polymer (or the softness of the polymer), the better the degree of compaction of the coating. As a result, the final coating structure will be more xe2x80x9cclosexe2x80x9d. This has a significant impact on the moisture barrier performance as will be observed hereinbelow.
According to the present invention, the preferred composition for use in providing a repulpable, chlorine free, wax free water vapor barrier coating comprises a tackifier resin, the above-defined polymer and filler The tackifier is preferably an aqueous dispersion of glycerol ester of hydrogenated rosin. One particular commercially available source is an aqueous dispersion of glycerol ester of hydrogenated rosin, which is sold under the trademark Staybelite ester 10.55WK by Hercules Inc. It is described as a dispersion in water having a content of rosin of about 55% (55%xc2x11%), a pH of 9.00 to 10.0, a viscosity at 25xc2x0 C. of 1450 cps to 3050 cps and a softening point of 73xc2x0 C. Rosin esters, rosin acids and mixtures of rosins and rosin derivatives may also be used within the scope of the present invention.
The polymer is preferably a carboxylated styrene butadiene copolymer dispersion. One commercially available source of carboxylated styrene butadiene copolymer dispersion is sold under the trademark Styronal BN 4606X by BASF and is described as a dispersion in water having a polymer content of about 50% (50%xc2x11%), a pH of 6.5 to 7,5, a Brookfield viscosity at 23xc2x0 C. of 350 cps and a glass transition temperature Tg of 6xc2x0 C. The dispersion is further described as being anionic and having a density of 1.01 g/cm3. Polyvinyl acetate, polyvinyl alcohol polyvinyl acetate-ethylene, polyvinyl acrylic and polyacrylic dispersions may also be used within the scope of this invention, under conditions that they are not to be softened by water. Styrene acrylic copolymer dispersions and acrylic polymer dispersions, which are water-resistant, may also be used according to this invention. The above mentioned acrylic copolymers are preferably derived from butyl acrylate, 2-ethyl hexyl acrylate and generally higher alkyl acrylates.
The above polymers may also contain functional groups such as COOH, CONH2, OH and the like, to enhance mechanical stability, wettability, runnability and adhesion on cellulose substrate. Runnability is defined as the ability to apply a coating formulation which maintain coat weight control without encountering significant coating defect.
The preferred filler is mica. One particular commercially available source of mica is sold under the trademark Alsibronz 32 by Engelhard Corporation, and is described as having an average particle size of 32xcexc, and a specific gravity of 2.8-3.0. While mica provides excellent results, other filler such as talc or any filler having a plate like structure can also be used according to the present invention.
The aqueous moisture barrier coating according to the invention comprises water in an amount sufficient to provide an aqueous dispersion of the tackifier resin and an aqueous dispersion of the polymer, and also to disperse the filler, when the latter is present in the composition. The repulpable chlorine free, wax free, water vapor barrier coating may additionally contain a dispersing agent which is used to disperse the filler into the aqueous phase. Dispersing agents are well known to those skilled in the art and their choice is left entirely to the expert in the field.
The repulpable water vapor barrier coating may also contain a thickening agent to aid in dispersing the filler into the aqueous phase. Again, the choice is entirely left to one skilled in the art. A buffer, such as ammonia, may also be added to the composition to adjust the pH to a value between about 7.0 and 10.0 and to more easily disperse the filler. Finally, a defoaming agent may be necessary to prevent any foaming during dispersion of the filler. The choice of defoaming agents is left entirely to one skilled in the art.
In general, the repulpable, chlorine free, wax free, water vapor barrier coating composition according to the invention, may comprise:
The foregoing invention having been described, the following examples are provided to further teach preferred embodiments and to aid those skilled in tile art in the practice of the invention.
In the following examples, the coating weights referred to are dry coating weight. The water transmission rate was determined using an instrument sold under the trademark Mocon.